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Photosystem I (PSI) forms a large macromolecular complex of ∼580 kDa that resides in the thylakoid membrane and mediates photosynthetic electron transfer. PSI is composed of 18 protein subunits and nearly 200 co-factors. The assembly of the complex in thylakoid membranes requires high spatial and temporal coordination, and is critically dependent on a sophisticated assembly machinery. Here, we report and characterize CO-EXPRESSED WITH PSI ASSEMBLY1 (CEPA1), a PSI assembly factor in Arabidopsis (Arabidopsis thaliana). The CEPA1 gene was identified bioinformatically as being co-expressed with known PSI assembly factors. Disruption of the CEPA1 gene leads to a pale phenotype and retarded plant development but does not entirely abolish photoautotrophy. Biophysical and biochemical analyses revealed that the phenotype is caused by a specific defect in PSI accumulation. We further show that CEPA1 acts at the post-translational level and co-localizes with PSI in nonappressed thylakoid membranes. In native gels, CEPA1 co-migrates with thylakoid protein complexes, including putative PSI assembly intermediates. Finally, protein–protein interaction assays suggest cooperation of CEPA1 with the PSI assembly factor PHOTOSYSTEM I ASSEMBLY3 (PSA3). Together, our data support an important but nonessential role of CEPA1 in PSI assembly. Arabidopsis CO-EXPRESSED WITH PSI ASSEMBLY1 interacts with PHOTOSYSTEM I ASSEMBLY3 and plays an important but nonessential role in assembly of photosystem I. [ABSTRACT FROM AUTHOR]

The conversion of light energy to chemical energy by photosynthesis requires the concerted action of large protein complexes in the thylakoid membrane. Recent work has provided fundamental insights into the three-dimensional structure of these complexes, but how they are assembled from hundreds of parts remains poorly understood. Particularly little is known about the biogenesis of the cytochrome b6f complex (Cytb6f), the redox-coupling complex that interconnects the two photosystems. Here we report the identification of a factor that guides the assembly of Cytb6f in thylakoids of chloroplasts. The protein, DE-ETIOLATION-INDUCED PROTEIN 1 (DEIP1), resides in the thylakoid membrane and is essential for photoautotrophic growth. Knock-out mutants show a specific loss of Cytb6f, and are defective in complex assembly. We demonstrate that DEIP1 interacts with the two cytochrome subunits of the complex, PetA and PetB, and mediates the assembly of intermediates in Cytb6f biogenesis. The identification of DEIP1 provides an entry point into the study of the assembly pathway of a crucial complex in photosynthetic electron transfer. The Cytb6f complex is a multi-subunit enzyme that couples the two photosystems during the light reactions of photosynthesis. Here the authors show that the thylakoid-localized DEIP1 protein interacts with the PetA and PetB subunits, and is essential for Cytb6f complex assembly in Arabidopsis. [ABSTRACT FROM AUTHOR]

The term "de-etiolation" refers to the light-dependent differentiation of etioplasts to chloroplasts in angiosperms. The underlying process involves reorganization of prolamellar bodies (PLBs) and prothylakoids into thylakoids, with concurrent changes in protein, lipid, and pigment composition, which together lead to the assembly of active photosynthetic complexes. Despite the highly conserved structure of PLBs among land plants, the processes that mediate PLB maintenance and their disassembly during de-etiolation are poorly understood. Among chloroplast thylakoid membrane-localized proteins, to date, only Curvature thylakoid 1 (CURT1) proteins were shown to exhibit intrinsic membrane-bending capacity. Here, we show that CURT1 proteins, which play a critical role in grana margin architecture and thylakoid plasticity, also participate in de-etiolation and modulate PLB geometry and density. Lack of CURT1 proteins severely perturbs PLB organization and vesicle fusion, leading to reduced accumulation of the light-dependent enzyme protochlorophyllide oxidoreductase (LPOR) and a delay in the onset of photosynthesis. In contrast, overexpression of CURT1A induces excessive bending of PLB membranes, which upon illumination show retarded disassembly and concomitant overaccumulation of LPOR, though without affecting greening or the establishment of photosynthesis. We conclude that CURT1 proteins contribute to the maintenance of the paracrystalline PLB morphology and are necessary for efficient and organized thylakoid membrane maturation during de-etiolation., Competing Interests: The authors declare no competing interest.